1. Field of the Invention
The present invention relates to compositions and structures of a new class of synthetic diamond and diamond-like particles and the novel methods for producing such particles. The invention enables the manufacturer to tailor the properties of the diamond particles, including particle composition, size, size distribution, shape, crystalline structure, surface properties and electrical resistivity.
As used in regard to the present invention, the term "diamond" encompasses the cubic and all hexagonal polytypes of crystalline sp.sup.3 carbon. The term "diamond-like" includes all carbon structures with short range order containing primarily sp.sup.3 bonded carbon.
2. Discussion of Background
There are currently no available methods for mass-producing diamond powder with high purity or controlled purity. Most commercially available diamond powder contains a high concentration of impurities which are either dissolved within the crystalline lattice or are second phase inclusions. Industrial grade, natural diamond powder is characterized by containing both types of impurities wherein the second phase inclusions are naturally occuring minerals and the dissolved impurity is usually nitrogen. Industrial grade, natural diamond powder is yellow in color due to the dissolved nitrogen and is classified type Ia. There are two additional classes of natural diamond, type IIa and type IIb. Type IIa diamond is extremely pure, colorless, and gem quality; type IIb diamond is boron-doped, blue in color and also of gem quality. Powder composed of type IIa or type IIb diamond is unsuitable for industrial applications because of their great cost.
There are two conventional methods presently used for producing synthetic diamond:
Diamond grit is synthesized by precipitation from a carbon plus metal solvent/catalyst solution at high temperatures (e.g. 2000.degree. C.) and high pressures (e.g. 30 kbar). This high temperature/high pressure (HT/HP) diamond can be free of second phase inclusions, but almost always contains significant concentrations of dissolved nitrogen and the metal solvent/catalyst. Typical solvent/catalyst metals include alloys of nickel, iron, and cobalt. The nitrogen content of this HT/HP yellow diamond classifies it as type Ib.
Diamond powder is also mass produced by shock wave synthesis in which an explosive charge shocks a mixture of carbon and a metal solvent/catalyst. The shock-produced diamond is typically contaminated with dissolved nitrogen and metal, usually iron. Elaborate chemical processing is required to recover the diamond particles from the surrounding graphite and metal within the reaction chamber.
Diamond powder can also be made by precipitation of diamond within certain amorphous metals which are saturated with carbon. Shingu et al. U.S. Pat. No. 4,485,080 describe a two step process for the rapid solidification of carbon-containing metal alloys, followed by the precipitation of diamond particles inside the amorphous metal at temperatures above 100.degree. C. The diamond must be recovered from the metal by acid digestion. Such diamond particles are probably highly impure.
All industrial methods for diamond powder synthesis require extraction of the diamond particles from their metal matrix, followed by comminution and sizing of the particles into the desired size ranges. The cost per gram of grading industrial diamond into size categories usually exceeds the cost of synthesizing the diamond grit by a factor of approximately four.
More recently, a third method of diamond synthesis has been introduced, namely, the deposition of diamond thin films from the gas phase by activated chemical vapor deposition (CVD). In most instances of diamond CVD, diamond particles nucleate on a surface (heterogeneously) and then the particles grow in size. The particles may be widely separated or may be close enough to coalesce into a continuous diamond film.
Previous research has reported that the nucleation of isolated diamond grains on specially prepared substrates may be controlled within certain parameters. However, the heterogeneous nucleation of diamond particles has two drawbacks as a method for diamond particle synthesis:
(a) The diamond particle will be contaminated by the substrate. If the goal is to synthesize high purity diamond, then a diamond substrate would be required for heterogeneous nucleation. If a diamond substrate were employed, however, the diamond particle would be tightly bonded to their substrate thereby making it difficult to collect the particles.
(b) The isolated diamond grains must be separated from their substrate. If the diamond particles are below 1 .mu.m in diameter, the isolation and extraction procedure becomes as cumbersome as the procedure currently employed to isolate synthetic diamond particles from their parent metal/solvent catalyst.
One example of deposition on a surface followed by particle extraction has been reported by Banks in U.S. Pat. No. 4,495,044. This patent describes the formation of "diamond-like flakes" by arc evaporation of graphite onto a vertical substrate. The evaporated carbon attains diamond-like structure by simultaneous bombardment of the substrate by an argon ion beam. The diamond-like film grows on the substrate until it attains a thickness resulting in excessive film stress causing the film to disintegrate into flakes which then fall into a collection bin. This method does not permit control of the size, shape, or chemical composition of the diamond flake. It is also to be expected that the flakes are probably chemically contaminated by material from the substrate. For these reasons, therefore, no disclosure is contained for the synthesis of true, crystalline diamond.
Holcombe, Jr. et al. (U.S. Pat. No. 4,228,142) claims methods for depositing "metastable carbon" (including diamond and diamond-like carbon) onto silicon carbide (SiC) particles by the pyrolysis of CF.sub.4. Specifically, the patent discusses a two-step procedure in which SiC particles are deposited onto a surface by the thermal decomposition of various gases, followed by the decomposition (plasma may be utilized to affect gas decomposition) of CF.sub.4 onto the surfaces of the SiC particles. Although the inventors disclose the use of catalyst metals (such as iron) as a preferred method of promoting the deposition of diamond and diamond-like phases onto the SiC particles, as well as the deposition of metastable carbon onto SiC particles in a fluidized bed, they do not disclose homogeneous nucleation of the SiC seeds, nor the sequential use of such SiC particles as seeds for diamond growth in a flowing gas stream.
All of the aforementioned methods of diamond particle synthesis suffer from: (a) the inability to create high purity, crystalline diamond, (b) the inability to controllably dope the diamond particles, and (c) the inability to directly synthesize particles of a desired size or size distribution, e.g. monosized particles.
A number of researchers have claimed to produce diamond in the gas phase. Most of them do not provide clear evidence that the diamond powder was homogeneously nucleated; in most cases, diamond particles were nucleated on surfaces. Recent publications by Soviet researchers present results indicating homogeneous nucleation of diamond powder, viz., V. T. Popov, et al., Colloid J. USSR, 49, 546(1987) and V. T. Popov, et al., Proc. Acad. Sci USSR Phys. Chem. Sect. 296, 923(1987). The Soviet researchers reported the formation of diamond powder in acetylene pyrolysis, after the products of thermal pyrolysis were quenched either by injection of water or by expansion of the gas in nozzle. Plasma oxidation of the material formed in the experiments resulted in a mixture of diamond and lonsdaleite particles about 20 nm in diameter. The Soviet authors did not provide details about the pyrolysis conditions.
Matsumoto et al. (U.S. Pat. No. 4,767,608) discloses methods for producing diamond thin films as well as diamond powders. Diamond powder formation results when a substrate is not present inside the reaction chamber. The patent describes the production of diamond powder in a plasma reactor by using suitable hydrocarbon-hydrogen-inert gas mixtures. Diamond powder homogeneously precipitates because of the adiabatic expansion of certain gases through an orifice into a plasma excitation zone. The patent does not, however, describe any methods to separately control the nucleation step and growth step for synthesizing diamond particles, nor does it consider the homogeneous precipitation of non-diamond seeds which promote the deposition of diamond to larger particle diameters. Furthermore, the patent does not contemplate methods to control the size, purity and crystal structure of the diamond particles.